Sørensen and the Invention of the pH Scale

The pH scale is one of the most familiar ideas in science. It appears on shampoo bottles, swimming pool test kits, soil reports and school exam papers worldwide. It’s easy to assume it has always existed. In fact, it was invented in a specific place, for a specific reason, by a specific person, and that place was a laboratory owned by a beer company.

The Carlsberg Laboratory

In 1875, the Danish brewer J. C. Jacobsen founded the Carlsberg Laboratory in Copenhagen. It was an unusual idea for its time: a private scientific institute, funded by a brewery, devoted to fundamental research in chemistry and physiology that could improve brewing. It became one of the most important research centres in the history of biochemistry.

Brewing is applied biochemistry. Enzymes in malted barley break down starch into sugars; yeast ferments those sugars into alcohol and carbon dioxide. Every step depends on conditions such as temperature and acidity. Brewers had long known that “sourness” mattered, but they had no good way to measure it.

Søren Sørensen

Søren Peder Lauritz Sørensen (1868–1939) became head of the Carlsberg Laboratory’s chemistry department in 1901. He was a careful, methodical chemist who studied proteins, amino acids and, above all, enzymes.

In his work on enzymes, Sørensen noticed something important. The activity of an enzyme depended strongly on the acidity of its solution. But “acidity” was being measured in inconsistent ways. Two solutions made with the same amount of acid could behave very differently if one acid was strong and the other weak, or if salts were present. What mattered, Sørensen realised, was the actual concentration of hydrogen ions.

The problem with the numbers

Hydrogen ion concentrations in real solutions span an enormous range, from around 1 mol/dm³ in strong acid to 0.00000000000001 mol/dm³ (10⁻¹⁴) in strong alkali. Writing and comparing numbers like 0.0000063 or 2.5 × 10⁻⁹ was awkward, especially in the days before calculators, and made it hard to see patterns.

In 1909, Sørensen published a paper on enzyme studies that introduced a neat solution. He defined a new quantity, the negative power of ten of the hydrogen ion concentration. He originally wrote it as pᴴ (with the H as a subscript or superscript); the form pH became standard later.

In modern notation:

pH = −log₁₀[H⁺]

So a solution with [H⁺] = 10⁻⁵ mol/dm³ has pH 5. Numbers that were unwieldy became simple values between about 0 and 14.

What does the “p” stand for?

Nobody knows for certain, and Sørensen didn’t clearly explain it. Popular suggestions include:

• “potenz” (German for “power”), since pH is a power of ten
• “puissance” (French for “power”)
• “potential”
• simply a mathematical label, since in his paper Sørensen used p and q for two electrodes, and the “p” may have come from that notation

The “power of hydrogen” explanation is the one most often repeated, but historians of science have pointed out that the evidence is thin. Today, chemists treat “p” as an operator meaning “−log₁₀ of”, which is why we also have pOH, pKa, pKb and pKw.

How Sørensen measured it

Sørensen used two methods:

1. Electrochemical measurement with a hydrogen electrode. The voltage between two electrodes in contact with a solution depends on the hydrogen ion concentration, following the equation derived by Walther Nernst in 1889. This was accurate but cumbersome.
2. Indicators. Sørensen prepared a series of buffer solutions of known pH and used them to calibrate the colour changes of indicator dyes. By matching the colour of a test solution with indicator to the standards, he could estimate its pH. Many of his buffer recipes (glycine, citrate and phosphate buffers) were used for decades.

His careful buffers were as important as the scale itself: they made pH measurement reproducible in any laboratory.

Enzymes and the optimum pH

With his new tool, Sørensen showed that each enzyme has an optimum pH at which it works best, with activity falling on either side. This was a major insight into how proteins work, and it’s why biochemists still control pH carefully in every experiment. See pH inside the body for how different body compartments keep different pH values for their enzymes.

Margrethe Sørensen

Sørensen’s wife, Margrethe Høyrup Sørensen, was also a chemist at the Carlsberg Laboratory, and the two collaborated on research into proteins, including lipoproteins and the behaviour of proteins in solution. Her contributions are less well known than they deserve to be, which is true of many women scientists of the period.

From concentration to activity

Chemists soon realised that in anything but very dilute solutions, ions don’t behave independently: they attract and repel each other, so their “effective concentration” (called activity) differs from their actual concentration. In 1924, Sørensen and Kaj Linderstrøm-Lang revised the definition to use the activity of hydrogen ions:

pH = −log₁₀ a(H⁺)

This is the definition used in precise work today. For dilute solutions, the difference is small, and the concentration version taught in schools works well.

The pH meter

Measuring pH became much easier with the invention of the glass electrode, based on the discovery (by Max Cremer in 1906 and developed by Fritz Haber and Zygmunt Klemensiewicz in 1909) that a thin glass membrane develops a voltage that depends on pH. In the 1930s, the American chemist Arnold Beckman built a practical, portable pH meter, reportedly first to help a citrus company measure the acidity of lemon juice. His company went on to become a major scientific instrument maker. See how a pH meter works.

pH spreads everywhere

Within a few decades, pH had spread far beyond brewing and biochemistry:

• Medicine: blood pH became a key clinical measurement.
• Agriculture: soil pH became central to farming (see soil pH and plant growth).
• Water treatment: pH control made drinking water safer.
• Food science: pH became essential for food safety and preservation.
• Environmental science: pH is used to monitor rain, rivers and oceans.

Why the story matters

The pH scale shows how a practical industrial problem, making consistent beer, led to a fundamental scientific tool. It also shows the value of good notation. The chemistry of hydrogen ions didn’t change in 1909; what changed was how easily scientists could think and talk about it.

Key takeaways

• Søren Sørensen introduced the pH scale in 1909 at the Carlsberg Laboratory in Copenhagen while studying enzymes.
• pH turned awkward hydrogen ion concentrations into simple numbers using a negative logarithm.
• The meaning of “p” is uncertain; today it’s used as shorthand for “−log₁₀”.
• Sørensen’s buffer solutions and his discovery of optimum pH for enzymes were as important as the scale.
• The definition was later refined to use hydrogen ion activity; Beckman’s pH meter made measurement routine.
• For the maths behind the scale, see the pH scale explained.